1. Field of the Invention
The present invention relates to communication systems which employ spread-spectrum signals and, more particularly, to receivers for detecting spread-spectrum signals in a Direct Sequence (DS) CDM communication system.
2. Background of the Invention
The wireless telecommunications industry provides the capability of telephony to a moving public through cellular phones. CDMA is a military-based digital communication system and a digital standard for cellular phone systems. A special case of the CDMA system is a spread-spectrum system, i.e. spread spectrum CDMA or SS/CDMA. In a spread-spectrum system, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band within the communication channel.
One type of spread-spectrum communication technique, direct sequence modulation, relates particularly to the present invention. In direct sequence (DS) modulation, a carrier signal is modulated by a digital code sequence whose bit rate is much higher than the information signal bandwidth. In direct sequence systems communication between two communication units is accomplished by spreading each transmitted signal over the frequency band of the communication channel with a unique user spreading code, hereinafter referred to as a signature or code. As a result, transmitted signals are in the same frequency band of the communication channel and are separated only by the unique user spreading codes or signatures.
Information (i.e. the message signal consisting of voice and/or data) can be embedded in the direct sequence spread-spectrum signal by several methods. One method is to add the information to the spreading code before it is used for spreading modulation. Alternatively, the information or message signal may be used to modulate a carrier after spreading it.
Particular transmitted signals can be retrieved from the communication channel by despreading a signal representative of the sum of signals in the communication channel with a user spreading code related to the particular transmitted signal which is to be retrieved from the communication channel.
Preferably, the user spreading codes are orthogonal to one another such that when the received signal is correlated with a particular user spreading code, only the desired user signal related to the particular spreading code is preserved, while the other signals for all of the other users are eliminated. In many cases the spreading codes are not orthogonal. This may occur by design or may exist due to the transmission characteristics of the channel.
Several different spreading codes are known which can be used to separate data signals from one another in a DS CDMA communication system. These spreading codes include but are not limited to pseudo noise (PN) codes and Walsh codes.
In a mobile cellular phone system, there are problems inherent in designing DS/SS CDMA receivers for either the mobile user or the base station with which it communicates. The base station must simultaneously detect and recover all signals of the known mobile users in its domain. Given that the optimum multiuser detector exhibits unrealizable complexity, i.e. exponential in the number of users and system processing gain, suboptimal linear-complexity solutions were sought. Some proposals include the decorrelating receiver, multistage architectures and decision feedback detectors.
The mobile users, however, have a more difficult problem. Knowing only its own signature, a mobile user's receiver must detect its own information bits in the presence of unknown spread spectrum, or multiple access (MAI) interference, and additive white Gaussian (AWG) channel noise.
Increasing the challenge of designing the mobile receiver is the fact that it must be of a desirable size and weight, i.e. lighter and smaller than would be acceptable for a base station receiver.
Demodulating a DS/SS CDMA signal in the presence of multiple access interference has been previously addressed in the prior art. One solution, the "matched filter (MF) solution exhibits performance degradation in the presence of one or more high power interferers, known as the "near-far" problem. This characteristic of MF receivers requires that they be used with some form of stringent, and costly, power control.
U.S. Pat. No. 5,345,472 to Lee discloses a DS/SS CDMA receiver for adaptively decoding DS/SS communication signals. In this system the CDMA transmitter transmits a training bit sequence and the receivers adaptively determine, based on the training sequence, the despreading codes. This is accomplished by converging or minimizing the error between the received training bit sequence and the reference bit sequences.
While this system allows all users to communicate with each other over a channel without requiring knowledge of system parameters, it disadvantageously requires transmission of a separate training sequence which places a concomitant processing burden on the receiver.
U.S. Pat. No. 5,343,496 to Honig et al. discloses circuitry and concomitant methodology for demodulating DS/SS CDMA channel signals using multiple samples per transmitted symbol and a minimum mean-square error criterion to suppress interference. Honig et al, propose to minimize the mean-square error (MSE) between the output of the filter and the desired information bit. However, this technique has the disadvantage of requiring a separate training sequence.